Angular position sensors, such as resolvers, are well known. A resolver is a position sensor that measures the angle or amount of rotation of a shaft. When calibrated properly, resolvers are capable of great precision.
Resolvers are used in such applications as determining the angular orientation of gimbaled devices. Examples of gimbaled devices include antennae and optical instruments. Line-of-sight antennae, such as those used in microwave and laser communications, require precise positioning. Optical instruments, such as telescopes and cameras, also require precise positioning.
Resolvers are frequently used in spacecraft to facilitate the pointing of antennae, optical instruments and the like at distant objects. Such space-based applications of resolvers present particular challenges, since according to contemporary methodology the ability to perform resolver system calibration after launch is limited or non-existent.
To point or position a gimbaled device, a computer typically sends a drive signal to a positioning motor that effects the desired movement of the gimbaled device. A resolver provides a position signal for the device, so that the difference between the device's actual position and its desired position can be determined. This difference is used according to well known principles to effect further movement of the device toward the desired position. Thus, resolvers are often important parts of position feedback loops that control the pointing of gimbaled devices.
Resolvers provide a sine signal and a cosine signal that, taken together, are indicative of the angle to which a shaft has been rotated. The sine signal comes from one winding, i.e., the sine winding, of the resolver and the cosine signal comes from another winding, i.e., the cosine winding, of the resolver.
The sine and cosine windings define two secondary windings of a transformer. They are typically part of a stator of the resolver, while a single primary winding is typically part of a rotor of the resolver.
The position of the rotor determines the amount of coupling of the primary winding with the two secondary windings. The coupling, and consequently the amplitude of the coupled signal, is therefore indicative of the position of the rotor.
The two secondary windings are oriented at approximately 90° with respect to one another. Thus, when one secondary winding has maximum coupling with respect to the primary winding, the other secondary winding has minimum coupling with respect thereto.
As the rotor turns, one of the two secondary windings produces a signal that can be considered a sine signal, while the other secondary winding produces a signal that can be considered a cosine signal since the two signals are 90° out of phase with respect to one another. Since the tangent of an angle is equal to the sine of the angle divided by the cosine of the angle, an arctangent function can be used to determine the angle of the shaft from the sine and cosine signals.
The outputs of the sine and cosine windings may be amplified and then digitized to provide digital angular position information for use by a digital processor. However, the precision of such contemporary resolver systems is undesirably limited by errors, such as those introduced by the use of multiple pole windings and by non-linearities of the resolver amplifiers.
Multiple pole windings are commonly used in contemporary resolvers. That is, instead of a single large sine winding and a single large cosine winding, a plurality of smaller sine and cosine windings, distributed about the rotor, are generally provided.
The positioning of the multiple sine and cosine windings with respect to one another must be exact in order to achieve perfect sine and cosine signals. However, due to inherent manufacturing tolerances, this positioning is never exact and undesirable errors are introduced. These multi-winding errors tend to reduce the precision of the resolver by introducing deviations in the sine and cosine outputs of the secondary windings.
Amplifiers, including the amplifiers of resolver positioning systems, are inherently non-linear to some degree. Of course, such non-linearity adversely affects the precision of a positioning system that uses the outputs of such amplifiers.
Contemporary methods of calibrating space-based resolver systems include both ground calibration procedures and mission operation calibration procedures. Ground calibration procedures are performed on the Earth, prior to launch. For example, a lookup table may be formed wherein error compensation or scale factors are provided for a plurality of different resolver system outputs. Thus, when a particular resolver system output is provided during mission operations, the appropriate scale factor can be applied to the resolver system output. Scale factors can be interpolated, when necessary.
Mission operation calibration procedures are performed in space, typically during the spacecraft's mission. For example, the gimbaled device can be moved to known positions, such as against gimbal stops, and then the scale factor lookup table can be updated.
However, such contemporary methods of calibrating space based resolver systems suffer from inherent drawbacks that substantially reduce their effectiveness and/or desirability. For example, ground procedures do not maintain the desired pointing precision over the life of the mission because resolver system errors tend to vary over time, due to such factors as temperature and mechanical wear.
Mission operation calibration procedures typically require undesirable mission interruptions in order to provide the necessary calibration data. Further, mission operation procedures are generally limited in their ability to provide adequate calibration data. For example, the number of known gimbaled device positions available for space based calibration is limited (typically, to those positions wherein the gimbals are against their stops).
As gimbaled devices such as antennae and optical instruments are used over greater and greater distances, the pointing precision required by their positioning systems substantially increases. As mission lifetimes increase in duration, the need to maintain such precision for longer periods of time increases.
As a result, there is a need for methods and systems to reduce the undesirable effects of errors in contemporary resolver systems and thereby enhance the utility of gimbaled devices and the like that rely upon such resolver systems for positioning.